This invention relates to a process and a catalytic composition for the preparation of aldehydes, i.e. aliphatic or aromatic compounds containing one or more --CHO groups, and to the aldehydes thus prepared.
Aldehydes may be prepared in a few general ways. Because of their ready availability, carboxylic acids would be advantageous starting compounds for the preparation of aldehydes. However, as detailed e.g., by J. D. Roberts and M. J. Caserio, "Modern Organic Chemistry", New York 1967, p. 310, conversion of a carboxylic acid to an aldehyde by direct reduction is not easy to achieve because acids are generally difficult to reduce, whereas aldehydes are easily reduced. Thus the problem is to keep the reaction from going too far.
For instance, as far as known, only lithium aluminum hydride is capable of mildly reducing carboxylic acids, esters or anhydrides; however, the reduction products are alcohols instead of aldehydes.
The most useful known aldehyde preparation procedures involve conversion of an acid to a derivative that either is more easily reduced than an aldehyde, or else is reduced to a substance from which the aldehyde can be generated. The so-called Rosenmund reduction involves the first of these schemes; in this procedure the acid is converted to an acyl chloride, which is reduced with hydrogen over a palladium catalyst to the aldehyde. The rate of reduction of the aldehyde to the corresponding alcohol has to be kept at a low level by poisoning the catalyst with sulphur. Reduction of an acid to a substance that can be converted to an aldehyde is usefully achieved by way of lithium aluminum hydride reduction of the nitrile corresponding to the acid. The reduction step is usually successful only if inverse addition is used; i.e., a solution of LiAlH.sub.4 has to be added to a solution of the nitrile, preferably at low temperatures, e.g., -50.degree. C., otherwise the reduction product is a primary amine. Clearly these known procedures are laborious, since they involve at least two steps, and usually more. Also, the use of chemicals like LiAlH.sub.4 may be quite convenient in a laboratory synthesis, but its use is less desired in industrial scale preparations, where straightforward hydrogenation would be much preferred. It is desirable therefore to provide a process and a catalytic composition for the preparation of aldehydes from carboxylic acids, or close derivatives thereof, in one step. More specifically, it is the object of the present invention to provide a process and a catalytic composition for the preparation of aldehydes, comprising the reduction of carboxylic acids or esters, anhydrides or salts thereof by hydrogenation.
According to the invention this is achieved for certain compounds of the general formula R'COOR when employing an alumina-based catalyst containing one or more rare earth metals and iron.